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Space Elevator

neal.fun

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Re: Space Elevator

#351

Post by btilly »

_carbyau_ wrote: post #349
nroets wrote: post #251

Earlier quoted context omitted.

The rocket fuel needed to produce that 40 MJ weighs close to 1 kg, especially when you include the oxidiser. So the energy needed to accelerate 1kg of payload to LEO velocity is much more.

That whole "tyranny of the rocket equation" thing is why I am surprised the actual first stage for launching a rocket is NOT a ground based reusable "up-chucker". Basically, I would have thought that any momentum that can be imparted to the rocket before it has to rely on its self propulsion would be a huge help. Not talking about eliminating self propulsion, just an assist so the rocket could carry a larger payload…

The structural support needed to keep the rocket from crumpling under the throw is extra weight to carry the rest of the way.

This is why Starship is aiming for a catch rather than legs. Legs that work at 1g added too much weight.

Re: Space Elevator

#352

Post by LorenPechtel »

shagie wrote: post #316
vee-kay wrote: post #306

Earlier quoted context omitted.

I just love such nerdy debates on HN on a hypothetical scenario/example. I think this thread would also be loved by the nerdy folks at https://Reddit.com/r/theydidthemath

If I recall correctly... my very first post on Reddit was doing calculations for a (practically immortal) person eating beans and storing the flatus for a trip to the moon (searching shows that this is a not-infrequent request). It was only concerned with quantity - not storage or the engine. ... and the source document for the numbers was based on a paper that is fairly easy to find given the proper keywords in goog…

Did you account for the weight of storage? Because I would think the tyranny of the rocket equation for such a low ISP would cause various levels of impossibility, not to mention the problem of getting enough thrust.

Re: Space Elevator

#353

Post by LorenPechtel »

aDyslecticCrow wrote: post #166
messe wrote: post #158

Earlier quoted context omitted.

> it's just hard to go up Going up is the comparatively easy part, it's not exactly rocket science. Going fast enough sideways so you stay up there is the tricky bit.

> Going fast enough sideways so you stay up there is the tricky bit. nah, thats the simple part. getting up there efficiently is the difficulty. once we're up, its just a matter of force over time to create a nice orbit. The faster you go, the more friction you face, and the more heat and vibration your equipment must endure. Going slower reduce friction and stress but use more energy just negating gravity. Slow rock…

Actually, not quite. You tolerate a bit of extra fire in exchange for reduced gravity loss. Going straight up until you clear atmosphere is not the minimum energy trajectory.

Re: Space Elevator

#354

Post by LorenPechtel »

_carbyau_ wrote: post #349
nroets wrote: post #251

Earlier quoted context omitted.

The rocket fuel needed to produce that 40 MJ weighs close to 1 kg, especially when you include the oxidiser. So the energy needed to accelerate 1kg of payload to LEO velocity is much more.

That whole "tyranny of the rocket equation" thing is why I am surprised the actual first stage for launching a rocket is NOT a ground based reusable "up-chucker". Basically, I would have thought that any momentum that can be imparted to the rocket before it has to rely on its self propulsion would be a huge help. Not talking about eliminating self propulsion, just an assist so the rocket could carry a larger payload…

Building your rocket to survive the upchucker costs more than the savings from being upchucked.

Chuckers are the optimal large scale solution for airless bodies, but they're horizontal. You spread the acceleration out over a very long distance so you don't need a super beefy spacecraft and your humans won't turn to goo. Basically, a maglev train except it has track above as well as below and it doesn't have a maximum speed. Wrap one around the lunar equator and it can eject anywhere from sundiver to interstellar escape with human-tolerable acceleration.

Re: Space Elevator

#355

Post by shagie »

LorenPechtel wrote: post #352
shagie wrote: post #316

Earlier quoted context omitted.

If I recall correctly... my very first post on Reddit was doing calculations for a (practically immortal) person eating beans and storing the flatus for a trip to the moon (searching shows that this is a not-infrequent request). It was only concerned with quantity - not storage or the engine. ... and the source document for the numbers was based on a paper that is fairly easy to find given the proper keywords in goog…

Did you account for the weight of storage? Because I would think the tyranny of the rocket equation for such a low ISP would cause various levels of impossibility, not to mention the problem of getting enough thrust.

It was based on “how long would it take to fill the tanks of a Saturn V?” … with the additional “let’s pretend we can 1:1 swap methane and kerosene.”

Re: Space Elevator

#356

Post by pfdietz »

LargoLasskhyfv wrote: post #343
pfdietz wrote: post #277

Earlier quoted context omitted.

Their Isp is very low, unfortunately, because the molecular weight of the combustion gas is too high. Ditto for oxygen/carbon monoxide. Maybe the Isp could be increased by mixing in some helium, but helium is very expensive.

Methalox should suffice for practical and technical reasons? Compensate the slight loss of ISP by using aerospiked rotating detonation engines...

Methalox contains hydrogen (methane is CH4), which turns into water, either in the engine, or oxidized by the atmosphere after being expelled.

Re: Space Elevator

#357

Post by LorenPechtel »

hinkley wrote: post #339
aDyslecticCrow wrote: post #166

Earlier quoted context omitted.

> Going fast enough sideways so you stay up there is the tricky bit. nah, thats the simple part. getting up there efficiently is the difficulty. once we're up, its just a matter of force over time to create a nice orbit. The faster you go, the more friction you face, and the more heat and vibration your equipment must endure. Going slower reduce friction and stress but use more energy just negating gravity. Slow rock…

The best way I’ve seen it described is that the first 9.8 m/s² of thrust only makes you hover in place. So the more g forces you generate the higher the efficiency of the engine - as long as thrust per kg of fuel doesn’t dip too far. This all changes in outer space, where 0.01g is a valid propulsion mechanism for long duration missions.

Yup. Look at the launch trajectory of the Webb telescope. The upper stage engine was too weak for the orbital insertion and the booster had to waste energy putting it higher than need be so the upper stage engine had altitude to trade for time to keep the telescope from hitting the atmosphere.

But it was the optimum solution because that engine had a long burn to take it to L2. Hauling less engine to L2 was worth more than the loss of the engine not being powerful enough to fight gravity.

Re: Space Elevator

#358

Post by LorenPechtel »

Retric wrote: post #287
CobrastanJorji wrote: post #261

Earlier quoted context omitted.

I don't think there are any physics reasons why it'd be impossible, but certainly we can't do it with existing technology. You'd need an air breathing jet that could get a vehicle to go about five or six times faster than any current such engine has ever achieved (i.e. around mach 20-30), which is perhaps ridiculous, but I don't think it's necessarily impossible, just something we don't know how to do. There have bee…

Basically when you cut thrust you must pass through that altitude again or escape orbit. So either fire a rocket in space to circularize the orbit or reach more than Earth’s escape velocity 25,020 mph (11.186 km/s, 40,270 km/h) ~ Mach 32.6, due to some drag in air to thin for any kind of air breathing engine to work. X-30 was aiming far lower ~Mach 20. Nuclear could make it more realistic than any form of chemical co…

Or go high enough to let the moon alter your orbit into one that doesn't hit the atmosphere.

Re: Space Elevator

#359

Post by LorenPechtel »

ianburrell wrote: post #315
Retric wrote: post #246

Earlier quoted context omitted.

You can reach space using air breathing jets. You can’t stay in space using air breathing jets.

There isn't enough air at high altitudes for jets to reach space even if you count 100km as space. The highest jet record is 37km in MiG-25. The scramjet record is 33km. I found source that says the limit is 40km at Mach 15.

Nobody has been able to build one, but I'm not aware of any proof that it's impossible. You need some way to build an engine that doesn't appreciably slow the air that's passing through it.

Re: Space Elevator

#360

Post by umeshunni »

sojournerc wrote: post #179
ddoolin wrote: post #165

Earlier quoted context omitted.

How do you pay with PayPal if not putting in your credit card or bank details? link is a pretty well-known online wallet and much simpler to use than PayPal.

I've never heard of link, so it's the difference between a random website and a well established brand. Not that hard to understand why someone might be hesitant to put in their cc details.

Link is owned by Stripe: https://link.com/terms/contracting-entity
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